1. Field of the Invention
The present invention relates to a piezoelectric type electroacoustic transducer, such as a piezoelectric receiver, a piezoelectric sounder, or other piezoelectric types of electroacoustic transducers.
2. Description of the Related Art
Conventionally, electroacoustic transducers such as piezoelectric sounders and piezoelectric receivers are used to generate alarm sounds or operating sounds in electronic devices or apparatuses, home electric appliances, portable telephones. In general, known electroacoustic transducers include a piezoelectric plate that is bonded to the surface of a metallic plate to provide a unimorph type vibrating plate, the peripheral portion of the metallic plate is fixed in a case, and an opening of the case is closed with a cover.
However, in the unimorph type vibrating plate, the piezoelectric plate, which is vibrated in the area expansion mode, is constrained by the metallic plate of which the area is not changed, such that the surface-flexural mode is caused. Therefore, the acoustic conversion efficiency is low. Furthermore, it is difficult to provide an electroacoustic transducer having a small size and a sound pressure characteristic with a low resonance frequency (e.g., see Japanese Unexamined Patent Application Publication No. 2001-95094 (Patent Document 1), Japanese Unexamined Patent Application Publication No. 2002-10393 (Patent Document 2), and Japanese Unexamined Patent Application Publication No. 61-30898 (Patent Document 3)).
Patent Document 1 discloses a piezoelectric vibrating plate having a high acoustic conversion efficiency. The piezoelectric vibrating plate is formed by laminating two or three layers of piezoelectric ceramics to form a laminate with an internal electrode being interposed between the layers, and forming main-surface electrodes on the front and back surfaces of the laminate. When an AC signal is applied between the main-surface electrodes and the internal electrode, the laminate is surface-flexural-vibrated. Thus, a sound is generated.
With the piezoelectric vibrating plate having the above-described structure, when an AC signal is applied between the main-surface electrodes and the internal electrode, the two vibrating regions (ceramic layers) arranged sequentially in the thickness direction are vibrated in opposite directions with respect to each other. Thus, the acoustic conversion efficiency of this piezoelectric vibrating plate is increased as compared to that of the unimorph type vibrating plate. This piezoelectric vibrating plate can generate a high sound pressure, and also, can be operated at a low frequency as compared to a unimorph type vibrating plate having the same size as the vibrating plate having the above-described structure.
The piezoelectric vibrating plate is primarily made of ceramics. Thus, the piezoelectric vibrating plate has a low drop-impact strength. Thus, according to the proposition by Patent Document 2, protecting films made of resin are provided on substantially the entire front and back surfaces a piezoelectric vibrating plate, such that the drop-impact strength is improved.
With the piezoelectric vibrating plates that are made of only piezoelectric ceramics as described above, the acoustic conversion efficiencies are high, but they have a very small thickness. Accordingly, the vibrating plates are often distorted or rippled. Moreover, the distortion does not occur in a constant direction. Therefore, when such a vibrating plate is supported in a box, the diameter of a circle which represents the node of the surface-flexural-mode is dispersed. Thus, the resonant frequency of the vibrating plate is substantially changed.
FIG. 10 shows a piezoelectric type electroacoustic transducer in which the piezoelectric vibrating plate is deflected. In FIG. 10, a piezoelectric vibrating plate A, a case B supporting the piezoelectric vibrating plate A, and a cover C are shown. The broken line in FIG. 11 represents the position of a node N of the surface-flexural-mode of the vibrating plate A.
When the piezoelectric vibrating plate A is warped upward, the distance L1 between the supporting points is increased as shown by the solid line in FIG. 10. On the other hand, when the piezoelectric vibrating plate A is warped downward, the distance L2 between the supporting points is decreased as shown by the broken line in FIG. 11. Each of the distances L1 and L2 between the supporting points is equivalent to the diameter L of a circle representing the surface-flexural-mode. Therefore, disadvantageously, when the plate is warped downward, the resonant frequency of the piezoelectric vibrating plate A is increased such that the sound pressure in a low frequency range is reduced.
The diameter of the circle representing the node of the surface-flexural-mode is dispersed depending upon the warping direction of the piezoelectric vibrating plate A. As a result, the resonant frequency of the vibrating plate is dispersed.